How Dk is measured, and is there a certifiable standard?

Oxygen permeability of contact lens materials is a crucial factor for the maintenance of ocular health for contact lens wearers. As a non-vascular tissue, the cornea depends on atmospheric oxygen supply to sustain normal metabolic processes. Lack of oxygen can lead to corneal edema and can compromise the natural defence mechanisms of the eye. Limbal redness has also been associated with insufficient oxygen supply to the ocular surface and can be a first sign of more serious health problems.

The full implications of these physiological mechanisms first became apparent in the early 1980s, when conventional hydrogel lenses were worn overnight for extended periods. With the additional restrictions on oxygen supply due to the closed eyelid and lack of tear exchange under the lens, corneal hypoxia lead to corneal swelling and adverse events. The landmark study by Holden and Mertz in 1984 (1) confirmed the correlation between oxygen supply and corneal swelling and established that a minimum of 87 X 10^-9 (cm ml O₂)/(sec ml mmHg) oxygen transmissibility is needed for hydrogel lenses to limit overnight edema to 4%.

Definitions

The inconsistent use of terminology relating to oxygen flow through a contact lens and the variability in the way oxygen flow is measured or calculated has caused great confusion amongst practitioners and patients. The physical properties oxygen flux (j), oxygen permeability (Dk) and oxygen transmissibility (Dk/t) are defined in a range of standards (see 2,3,4 and Table 1) and are the terms most often quoted. Oxygen transmissibility is a laboratory measurement that relates to a specific lens type with a particular thickness, whereas oxygen permeability is a material specific property that is calculated from lens oxygen transmissibility. A factor that has contributed to the confusion surrounding the differences between Dk and Dk/t is that once the oxygen permeability of a particular lens material is known, then the oxygen transmissibilities of lenses made from this material can be calculated by dividing the Dk by the thickness of each lens.

* under specified conditions

Because oxygen transmissibility is of greater clinical relevance, it is often quoted when comparing different lens types. Oxygen transmissibility calculated using the centre thickness of a –3.00 D lens is the value most commonly used by industry. However even under this convention, comparisons can be misleading as differences in peripheral thickness profiles and lens diameters are not accounted for, even though they can significantly affect oxygen supply to the cornea and the limbus. Even more confusingly, differences in the way Dk/t is measured can cause significant variation in reported Dk values and hence variation in calculated Dk/t for the same lens type.

Measurement Methods

The polarographic and coulometric techniques are the most commonly used methods of measuring oxygen transmissibility, and oxygen permeability is calculated by multiplying the Dk/t of a lens with the mean thickness of the measured area. Typically, the measurement area is the central 6 to 8 mm diameter zone of a –1.00 D lens. For both methods, the contact lens is clamped horizontally between two discs, creating an upper chamber which is filled with oxygen saturated PBS (phosphate buffered saline), and a lower chamber where the perfused oxygen is detected. The technique of oxygen detection is different between the two methods. In the coulometric method, a nitrogen carrier gas flows underneath the lens and transports the permeated oxygen to the oxygen detector. With the polarographic method, the oxygen detector is a Clark-type electrode, which is in direct contact with the posterior lens surface.

Both methods require careful control of all measurement and environmental conditions to achieve reliable results. While factors like sample temperature, clamping force and oxygen concentration in the PBS can be kept constant with diligent instrument design and meticulous use, the measurement errors due to edge and boundary layer effects, and lens dehydration cannot be avoided. These measurement errors can lead to inconsistencies between measurements of the same lens type, and particularly with lenses made from highly oxygen permeable lens materials.

A new measurement method claiming far superior sensitivity was recently proposed by Oberndorf (5). Although details are currently limited, this method uses an oxygen detector based on HPLC-EC (high performance liquid chromatography with electrochemical detection) and is to be used in combination with a patented sample application system (6). The methodology offers superior accuracy, no lens dehydration, short measurement time and minimal edge and boundary layer effects, making it applicable to the full range of RGP, hydrogel and silicone hydrogel lens materials. However as with every new technique, independent verification is needed and this will require full details of the procedures and instrument design to be published. The possibility patent infringement may also be an obstacle to the development of the method for research or commercial purposes.

Standards

The polarographic and coulometric methods are described in ANSI and ISO standards (2,3,4) which are virtually identical, with respect to methods and application. Both limit the application of the polarographic method to lens materials with oxygen permeabilities of less than 100 Barrer and also specify that the coulometric method is not to be used for hydrogel materials. The coulometric method is not suitable for hydrogel lenses because the posterior surface of the lens is exposed to air rather than a solution, which causes dehydration of the lens material results in a high level of variability in measurements. This leaves the measurement of Dk for recently developed high Dk silicone hydrogel contact lenses outside the scope of any internationally recognized standard. This is clearly an unacceptable situation, given the clinical importance and the emergence of ever more new materials falling within this category.

Conclusion

Even in a best case scenario, it will be many years before a new procedure such as the Oberndorf method is accepted as the ISO standard for oxygen permeability and transmissibility for contact lenses. In the interim, practitioners, patients and the contact lens industry would benefit if all publicized Dk or Dk/t data were accompanied by a footnote, stating typical error margins and the method used to obtain these values. Additionally, profile maps of the range of oxygen transmissibilities across plus and minus 3.00 D lenses would allow practitioners to assess the impact of lenses across the whole cornea, and at the limbus.

References

1. Holden BA, Mertz WG. Critical oxygen levels to avoid corneal edema for daily and extended wear contact lenses. Invest Ophthalmol Vis Sci 1984; 25: 1161-1167.
2. ISO International Standard 9913-1. Determination of Oxygen Permeability and Transmissibility by the Fatt Method. Geneva, Switzerland: International Organization for Standardization, 1996.
3. ISO International Standard 9913-2. Determination of Oxygen Permeability and Transmissibility by the Coulometric Method. Geneva, Switzerland: International Organization for Standardization, 2000.
4. American National Standards Institute. American National Standard for Ophthalmics-Contact Lenses-Standard Terminology, Tolerances, Measurements, and Physical Properties, ANSI Z80:20-1998, Section 8.18. New York: ANSI, 1996.
5. ISO International Standard ISO/TC 172/SC 7, Proposal for a new work item document N 598. Contact Lenses – Determination of oxygen transmissibility with the chromatographic oxygen sensor. March 2003
6. Austrian Patent 406712, 1999
7. Morgan CF, Brennan NA, Alvord L. Comparision of the coulometric and polarographic measurement of a high-Dk hydrogel. Optom Vis Sci 2001; 78: 19-29